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The Na/V/P/N(C 2 H 5 ) 3 /H 2 O system at 473 K and the new open framework structure of the vanadium phosphate phase Na v [((V 4−w 4+ V 1+w 5+ ) O 9 )(PO 4 ) 2 ] · (PO 4 ) x · (OH) y · z H 2 O (with v = 2.8 to 4.0, w = −0.1 to 1.1, x = 0 to 0.2, y = 0 to 2.1, and z = 7 to 10)
Author(s) -
Schindler Michael,
Joswig Werner,
Baur Werner H.
Publication year - 1997
Publication title -
zeitschrift für anorganische und allgemeine chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.354
H-Index - 66
eISSN - 1521-3749
pISSN - 0044-2313
DOI - 10.1002/zaac.19976230108
Subject(s) - crystallography , vanadium , microporous material , crystal structure , chemistry , coordination number , hydrothermal synthesis , natural bond orbital , hydrothermal circulation , inorganic chemistry , molecule , ion , organic chemistry , seismology , geology
An examination of the Na/V/P/N(C 2 H 5 ) 3 /H 2 O system at 473 K under hydrothermal conditions yields several different phases in the pH range of 2.65 to 12. At low pH values a NaV 4+ OPO 4 phase was obtained. In the weakly acid, neutral or low basic pH ranges the Na v [((V 4−w 4+ V 1+w 5+ ) O 9 )(PO 4 ) 2 ] · (PO 4 ) x · (OH) y · zH 2 O (or FVP‐1 for short) phase was found (with v = 2.8 to 4.0, w = −0.1 to 1.1, x = 0 to 0.2, y = 0 to 2.1 and z = 7 to 10). Its crystal structure was solved in space group Im 3 m with a = 16.025 Å. It contains V 4 4+ V 5+ O 9 (PO 4 ) 2 groups in which the vanadium atoms have a square pyramidal coordination of oxygen atoms in the shape of a spiked helmet, where the spike is the vanadyl group of the central V 5+ coordination polyhedron. These groups are statistically arranged around a mirror plane yielding an V 10/2 O 18/2 (PO 4 ) 2 grouping and connect to form an open porous framework by bridging via the phosphate groups following the topology of the NbO type net. The framework has a mean free channel diameter of about 4.1 Å and a density of 10.2 vanadium and phosphorus polyhedra per 1000 Å 3 . This makes it, together with two other recently found related frameworks, the most open microporous framework described so far. No aluminosilicates or aluminophosphates of similar openness are known. In air the FVP‐1 phase does not collapse when heated to 523 K, in vacuum it remains stable to 623 K. It dehydrates and rehydrates reversibly. Redox titrations of 19 samples show that the V 4+ /V 5+ ratios vary from 4.1(2)/0.9(2) to 2.9(2)/2.1(2). Syntheses using V 2 O 5 as a source of vanadium yield samples with lower V 4+ /V 5+ ratios than those based on VOSO 4 . The fact that FVP‐1 displays disorder makes the compound possibly interesting for potential catalytic applications.